Windows XP stuck on loading screen
If windows hangs at the loading screen it is probably hung on a custom driver. If you can get in safe mode, great, run msconfig and under "BOOT.INI" select the option "/NOGUIBOOT" under boot options and restart. You should see a list of drivers being loaded instead of the normal Windows logo. When it freezes, take note of what driver is trying to be loaded, then use google to look it up. It most likely corresponds to a third party program that you should uninstall.
Susprisingly Yahoo! Answers had a suggestion that seems valid.
It makes you "reset" Windows but leaves your programs and settings be. Probably your best bet.
XP REPAIR INSTALL - print this for easier use. Use of an OEM disk will probable fail. You need an authentic Windows XP Home or Pro disk for this repair.
Boot the computer using the XP CD. You may need to change the boot order in the system BIOS. Check your system documentation for steps to access the BIOS and change the boot order.
When you see the "Welcome To Setup" screen, you will see the options below. This portion of the Setup program prepares Microsoft Windows XP to run on your computer: To setup Windows XP now, press ENTER. To repair a Windows XP installation using Recovery Console, press R. To quit Setup without installing Windows XP, press F3.
Press Enter to start the Windows Setup. do not choose "To repair a Windows XP installation using the Recovery Console, press R", (you do not want to load Recovery Console). I repeat, do not choose "To repair a Windows XP installation using the Recovery Console, press R".
Accept the License Agreement and Windows will search for existing Windows installations.
Select the XP installation you want to repair from the list and press R to start the repair.
Setup will copy the necessary files to the hard drive and reboot. Do not press any key to boot from CD when the message appears. Setup will continue as if it were doing a clean install, but your applications and settings will remain intact.
Reapply updates or service packs applied since initial Windows XP installation. Please note that a Repair Install using an Original pre service pack 1 or 2 XP CD used as the install media will remove SP1/SP2 respectively and service packs plus updates isssued after the service packs will need to be reapplied.
Source(s): Windows XP repair site on the Internet and personal experience with repairing Windows XP
It seems that having done too much, you are almost out of options.
You can try using a trouble-shooting boot cd, such as the Ultimate Boot CD for Windows.
You can try to use your boot cd to roll Windows back to a previous restore point.
If you can't find a solution, you might have to do a repair installation as described by Ivo, and if this doesn't work (horror) then a full installation.
Before you try a repair I'd recommend booting to the Last Known Good configuration.
From the linked article:
The Last Known Good Configuration feature restores registry information and driver settings that were in effect the last time the computer started successfully.
This is less drastic than a Repair and you can always try a repair afterwards.